G Model SNA-8109; No. of Pages 7 ARTICLE IN PRESS Sensors and Actuators A xxx (2013) xxx–xxx Contents lists available at SciVerse ScienceDirect Sensors and Actuators A: Physical jo urnal homepage: www.elsevier.com/locate/sna High temperature micro-glassblowing process demonstrated on fused quartz and ULE TSG ∗ D. Senkal, M.J. Ahamed, A.A. Trusov , A.M. Shkel MicroSystems Laboratory, 4200 Engineering Gateway Building, Department of Mechanical and Aerospace Engineering, University of California, Irvine, CA 92697, United States a r t i c l e i n f o a b s t r a c t Article history: We report, for the first time, a MEMS fabrication process for building atomically smooth, symmetric 3- Received 17 August 2012 D wineglass and spherical shell structures, using low internal loss materials, namely fused quartz and Received in revised form ultra low expansion titania silicate glass (ULE TSG). The approach consists of three major steps: (1) a 27 November 2012 deep fused quartz cavity etch, (2) plasma activated bonding of fused quartz to fused quartz or TSG and Accepted 28 November 2012 ◦ (3) a high temperature (up to 1700 C) micro-glassblowing process. An in-house process capability of Available online xxx ◦ ◦ 1800 C glassblowing with a rapid cooling rate of 500 C/min was developed. Feasibility of the process has been demonstrated by fabrication of fused quartz and TSG micro-glassblown structures. Spheri- Keywords: Micro-glassblowing cal and inverted-wineglass shells with self-aligned stem structures were fabricated using this process. The approach may enable new classes of TSG and fused quartz MEMS devices with extremely low sur- 3-D MEMS Fused quartz face roughness (0.23 nm surface average), intrinsically low thermoelastic dissipation (QTED > 5E+10), and Ultra low expansion titania silicate glass highly symmetric structures (radial error < 500 ppm). © 2013 Elsevier B.V. All rights reserved. 1. Introduction thermoelastic dissipation (TED) have not yet been demonstrated in the literature. To take full advantage of the 3-D wineglass architec- Maximization of the Q-factor is key to enhancing performance ture, fabrication techniques with low surface roughness as well as of vibratory MEMS devices in demanding signal processing, timing materials with high isotropy and low TED are desired. and inertial applications [1]. Current MEMS fabrication techniques It has been demonstrated that single-crystal silicon MEMS limit the maximum achievable Q-factor by restricting the mate- devices can reach the fundamental QTED limit by using a combi- rial choice to few materials and device geometry to 2-D planar nation of balanced mechanical design and vacuum packaging with structures. Available materials such as single-crystal silicon have getters [7]. TED is caused by local temperature fluctuations due to relatively high thermoelastic dissipation and 2-D planar devices vibration and the associated irreversible heat flow, which results in are mostly limited by anchor losses. The macro-scale Hemispher- entropic dissipation. TED can be reduced either by decoupling the ical Resonator Gyroscope (HRG) with Q-factors over 25E+6 [2] frequencies of mechanical vibrations from the thermal fluctuations motivates the investigation of 3-D fused quartz micro-wineglass or by using materials with low coefficient of thermal expansion structures for use as vibratory elements in MEMS applications. This (CTE). This paper focuses on materials with low CTE, such as fused ◦ paper investigates the intriguing possibility of fused quartz micro- quartz (0.5 ppm/ C) and ultra low expansion titania silicate glass ◦ glassblowing as a means to fabricate 3-D shell micro-devices, Fig. 1. (0.03 ppm/ C), which can provide a dramatic increase in fundamen- With the emergence of novel fabrication techniques batch fab- tal QTED limit (QTED > 7E+10 for a TSG wineglass structure). When rication of 3-D wineglass structures are becoming possible. For compared to silicon, titania silicate glass and fused quartz dry etch- instance, hemispherical shells fabricated by deposition of polysil- ing suffers from an order of magnitude higher surface roughness, ® icon [3] or silicon nitride [4] thin films into isotropically etched lower mask selectivity, ∼1 :1 for KMPR photoresist and lower cavities have recently been demonstrated. Alternative fabrication aspect ratio, <5:1 [8,9]. ◦ techniques include “3-D SOULE” process for fabrication of mush- Pyrex glassblowing at 850 C on a silicon substrate was room and concave shaped spherical structures [5], as well as previously demonstrated for fabrication of smooth, symmetric blow molding of bulk metallic glasses into pre-etched cavities 3-D structures [10–13]. However, fused quartz/TSG glassblowing ◦ [6]. MEMS wineglass resonators with atomic smoothness and low requires temperatures upwards of 1600 C due to its higher soft- ening point, which prevents the use of fabrication processes that rely on a silicon substrate. This paper explores the hypothesis ◦ ∗ that high temperature glassblowing (up to 1700 C), may serve Corresponding author. Tel.: +1 949 824 6314. E-mail address: [email protected] (A.A. Trusov). as an enabling mechanism for wafer-scale fabrication of TSG and 0924-4247/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.sna.2012.11.040 Please cite this article in press as: D. Senkal, et al., High temperature micro-glassblowing process demonstrated on fused quartz and ULE TSG, Sens. Actuators A: Phys. (2013), http://dx.doi.org/10.1016/j.sna.2012.11.040 G Model SNA-8109; No. of Pages 7 ARTICLE IN PRESS 2 D. Senkal et al. / Sensors and Actuators A xxx (2013) xxx–xxx Fig. 1. Optical photograph of glassblown TSG inverted-wineglass structure. Outer ◦ diameter is 4200 ␮m. Glassblown at 1650 C. fused quartz 3-D wineglass structures. The approach consists of a high temperature micro-glassblowing process and an inverted- wineglass architecture that provides self-aligned stem structures. ◦ An in-house process capability of 1800 C glassblowing with a rapid ◦ cooling rate of 500 C/min was developed. Feasibility of the pro- cess has been demonstrated by fabrication of TSG and fused quartz micro-wineglass structures and spherical shells [14]. In the following sections, firstly the design parameters for micro-glassblowing is presented. This is followed by details of the fabrication process and the custom-designed fabrication equip- ment. The paper concludes with analysis of the glassblown structures and discussion of the results. 2. Design parameters This section focuses on analysis of geometric design parameters and the effect of material choice on fundamental Q-factor limits. In order to establish the relationships between material choice, device dimensions and Q-factor, parametric Finite Element Analysis (FEA) ® was conducted using Comsol Multiphysics Package [15]. 2.1. Geometric design Micro-glassblowing process utilizes an etched cavity on a sub- strate wafer and a glass layer that is bonded on top of this cavity, creating a volume of trapped gas for subsequent glassblowing of self-inflating spherical shells. When the bonded wafer stack is heated above the softening point of the structural glass layer, two effects are activated at the same time: (1) the glass layer becomes viscous, and (2) the air pressure inside the pre-etched cavity increases above the atmospheric level. This results in plastic deformation of the glass layer, driven by gas pressure and surface tension forces (glassblowing). The expansion of air (and hence the formation of the shell) stops when the pressure inside and outside of the glass shell reaches an equilibrium, creating a self-limiting process. During this deformation, the surface tension acting on the now viscous glass layer works towards minimizing the sur- Fig. 2. ULE TSG/fused quartz micro-glassblowing process, consists of: (a) footprint face area of the structure as a result a highly symmetric spherical of the desired geometry is patterned on fused-quartz, (b) fused quartz is etched shell with low surface roughness forms. The process allows simul- and TSG device layer is bonded on top, (c) TSG/fused quartz stack is glassblown ◦ taneous fabrication of an array of identical (or different if desired) at 1600–1700 C, (d) wineglass structure is released along the perimeter, and (e) fabricated wineglass structure (left) and spherical shells (right). shell structures on the same substrate. Shape and size of the final glassblown structure can be designed by changing starting conditions such as thickness of the device example a circular cavity creates a spherical shell when glassblown, layer, cavity shape and dimensions as well as environment temper- Fig. 2 (right). Inverted-wineglass structures can also be fabricated ature and pressure during glassblowing [10]. For example, changing by defining a central post inside the etched cavity, Fig. 2 (left). When the cavity diameter directly affects the diameter of the final glass- the device layer is bonded to this central post, it acts as an anchor blown shell, whereas changing the cavity depth (or volume) affects point, which allows the glassblown shell to fold around it, creating the height of the glassblown structure. It is also possible to fabricate a self-aligned stem structure as in Fig. 2(e). entirely different geometries by changing the initial conditions, for Please cite this article in press as: D. Senkal, et al., High temperature micro-glassblowing process demonstrated on fused quartz and ULE TSG, Sens. Actuators A: Phys. (2013), http://dx.doi.org/10.1016/j.sna.2012.11.040 G Model SNA-8109; No. of Pages 7 ARTICLE IN PRESS D. Senkal et al. / Sensors and Actuators A xxx (2013) xxx–xxx 3 Fig. 3. Wineglass structures with (a) 1.2 mm outer diameter and 600 ␮m stem diameter (hollow), (b) 4200 ␮m outer diameter and 500 ␮m stem diameter (solid) and (c) 4200 ␮m outer diameter and 150 ␮m stem diameter (solid). 2.2.